1. Fields of the Invention
The presenting invention relates to a medium access control (MAC) protocol for optical-fiber networks which use time-division multiple access (TDMA) technique to provide medium-sharing environment. The MAC uses traffic control methods thereof in which the medium access of optical networks can be logically and effectively distributed among nodes. Otherwise, a traffic control method is introduced and applied in the MAC. Because the presenting invention can improve the unfair distribution of access among nodes, the TDMA optical network can be applied to construct Metropolitan Area Networks (MANs) and Local Area Networks (LANs), and also can be considered as subnetworks of public networks so as to reduce construction cost and optimize bandwidth utilization and enhance communicative performance.
2. Descriptions of Related Art
In accordance with Hartley-Shannon theorem (the channel/information capacity theorem), channel capacity of transmission media is restricted by the bandwidth of transmission media and the signal-to-noise ratio. Media with low noise and wide bandwidth can support high capacity. Compared with twisted pairs, optical fibers have very low noise and wide bandwidth so that the capacity of optical fibers is much higher than that of twisted pairs. Due to prosperous development of various services, an urgent increase on capacity and communicative performance dramatically presents. Thus, twisted pairs have been gradually replacing by optical fibers in public communication networks.
Medium-sharing networks naturally have high bandwidth utilization. Their topologies are so simple that low constructional cost will be taken easily. Every variety of medium access control (MAC) protocol, such as carrier sense multiple access (CSMA), carrier sense multiple access with collision detection (CSMA/CD), token ring, token bus, time-division multiple access (TDMA) and so on, is exploited by medium-sharing networks to control access among nodes. The TDMA protocol is appropriate for supporting optical medium-sharing networks. On TDMA networks, there is no collision. Therefore, the bandwidth utilization of TDMA networks can approximate to its medium capacity. Though control protocols with tokens are also adopted to establish the optical medium-sharing environment, their packet delays are larger than that of TDMA systems. However, the optical TDMA network has an unfair-access problem due to its topology. The unfair problem indicates that the access of upstream nodes will be higher than that of downstream nodes. The problem had been fervidly explored before and after the IEEE 802 committee recommended optical TDMA networks to form the IEEE 802.6 LAN/MAN protocol, which is named distributed-queue dual-bus networks (DQDB networks), in 1990. Many approaches were proposed to improve the unfair problem, but it is yet to be resolved and has been laid for more than ten years. Because users within the optical TDMA network may be independent of one another, the unfair distribution of access among nodes becomes very troublesome for establishing the optical medium-sharing environment.
The unfair distribution of access influences the distributed-queue delay of DQDB nodes. In order to understand the characteristics of distributed-queue delays of DQDB nodes, several approaches were taken to analyze DQDB networks. All these analyses are based on the model Bisdikian proposed. Bisdikian introduced an approximate single-node analytical model. Due to the model, the steady-state generation function of the number of requests queued ahead of an arriving packet is expressed. Given this number, the distributed-queue delay of an arriving packet can be easily obtained. In a word, these approaches analyze the distributed-queue delay of every node according to the detailed operations of the MAC protocol of the network. Because the MAC protocol of the network is so complex, the modeling and performance analysis of the network become very difficult. To make an exact analysis on the distributed-queue delay by considering detailed operations of the MAC protocol is almost impossible. Consequently, these analyses cannot make a contribution to solve the unfair-access problem.
From the perspective of TDMA networks, the distributed-queue delay on DQDB networks is the same as the waiting time on TDMA networks. The waiting time of TDMA networks is a time interval. The start of the time interval is the instance that a packet going to be transmitted enters the nodal packet buffer attached to a bus transmission system. The end of the interval is the moment the packet is written into a free slot appearing on the bus transmission system. This definition is similar to that of the delay of distributed queues on DQDB networks. So, both the distributed-queue delay on DQDB networks and the waiting time on TDMA networks have same properties. If the waiting time on TDMA networks depends on network topology, i.e. the average waiting time of TDMA nodes (the waiting mean) is functions of nodal positions, optical TDMA networks would inherently accompany the unfair-access problem. When the exactness of the inherent property can be verified, to completely solve the unfair-access problem of optical TDMA networks should be impossible. Otherwise, this unfair-access problem can be solved after the waiting time on TDMA networks is analyzed. Under this concept, this invention first analyzes the waiting time on TDMA networks to distinguish whether the unfair-access problem can be completely solved or not.
In a stable TDMA network, the carried load must be equal to the offered load whichever the MAC protocol is used. Thus the waiting time of TDMA networks can be analyzed by the use of TDMA slots on media regardless of the operation of MAC protocols. Based on the observation of the use of TDMA slots, it is obvious that the waiting time of a packet generated by a node is dependent on the probability that the next available slot appears for the node. The probability can be determined by the traffic distribution among nodes and the capacity of the transmission medium.
Based on the probability that the next available slot appears for a node, the waiting mean of TDMA nodes has been analyzed. The analysis shows that the waiting mean is a simple function of nodal traffic. The waiting mean of the node is in inverse proportion to nodal traffic. This means that nodes with larger traffic will have lower waiting means. According to the analysis, if MAC protocols have the ability of traffic control, waiting means are irrelevant to nodal positions. This represents that the unfair-access problem will not exist if the MAC protocol of an optical TDMA network performs traffic control.
Thus a MAC protocol performing traffic control is proposed to control access among nodes of an optical TDMA network. Due to the MAC, the unfair distribution of nodal access on optical TDMA networks can be solved completely.